For over three decades, battery scientists have been frustrated with the low energy density of lithium-ion cells primarily due to the low lithium-storing capacity of all existing cathode active materials. Specifically, the practical specific capacity achievable with current cathode materials has been limited to the range of 150-250 mAh/g (mostly less than 200 mAh/g), leading to an energy density (specific energy) of 120-180 Wh/kg.
Presumably rechargeable lithium metal batteries featuring a lithium metal anode can exhibit a higher energy density (e.g. up to 250 Wh/kg for a polymer electrolyte Li cell), but lithium metal anode suffers from the severe dendrite problem, which has been a major safety issue in battery industry for more than 3 decades. In order to overcome this dendrite issue, most of the commercially available Li-ion cells make use of a carbon- or graphite-based material as an anode active material to replace bare Li metal foil. However, these carbon or graphite anode materials have several significant drawbacks: low specific capacity (theoretical capacity of only 372 mAh/g of graphite), slow Li intercalation (due to low solid-state diffusion coefficients of Li in graphite) resulting in a long recharge time, inability to deliver high pulse power, and necessity to use lithiated cathodes (e.g. lithium cobalt oxide), thereby limiting the choice of available cathode materials. Furthermore, these commonly used cathodes also rely upon extremely slow Li diffusion in the solid state. These factors have contributed to the two major shortcomings of today's Li-ion batteries—a low energy density (typically 120-180 Wh/kgcell) and low power density (˜0.5 kW/kgcell). It may be further noted that Li-sulfur and Li-air can exhibit a higher energy density, but the power density is even lower than that of Li-ion cells.
Hence, there has been strong and continued demand for batteries capable of storing more energy (Wh/l or Wh/kg) and delivering more power (W/kg or W/l) than current rechargeable Li-ion batteries. One possible route to meeting this demand is to utilize divalent magnesium ion (Mg2+), rather than the monovalent cation lithium (Li+) because magnesium enables nearly twice as much charge to be transferred, per weight or volume, as Li+ thus enabling high energy density. Further, magnesium metal and Mg-containing alloys or compounds are more abundant and readily available, potentially enabling significant cost reduction relative to Li-ion batteries. Unfortunately, in general, the cathode active materials capable of storing Mg ions exhibit even lower specific capacity (typically <200 mAh/g and more typically <150 mAh/g) as compared to the current cathode active materials for lithium-ion cells.
The cathode active materials proposed for use in a Mg-ion cell include: Chevrel phase Mo6S8, MnO2, CuS, Cu2S, Ag2S, CrS2, and VOPO4; layered compounds TiS2, V2O5, MgVO3, MoS2, MgV2O5, and MoO3; Spinel structured compounds CuCr2S4, MgCr2S4, MgMn2O4, and Mg2MnO4; NASICON structured compounds MgFe2(PO4)3 and MgV2(PO4)3; Olivine structured compounds MgMnSiO4 and MgFe2(PO4)2; Tavorite structured compound Mg0.5VPO4F; pyrophosphates TiP2O7 and VP2O7; and FeF3. For a review on the state-of-the-art of rechargeable Mg batteries, one may consult the following references:    (1) E. Levi, et al., “On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials,” Chemistry of Materials, 22 (2010) 860-868. (This reference provides a brief review on cathode active materials for rechargeable Mg cells.)    (2) Y. NuLi, et al. “Electrochemical Intercalation of Mg2+ in Magnesium Manganese Silicate and its Application as High-Energy Rechargeable Magnesium Battery Cathode,” J. Physical Chem., C. 113 (2009) 12594-97. (This reference provides an intercalation compound as a cathode active material.)    (3) J. Muldoon, et al., “Electrolyte roadblocks to a magnesium rechargeable battery,” Energy & Environ. Sci., 5 (2012) 5941-5950. (This reference provides perspectives on electrolyte-related issues of Mg cells.)    (4) D. Aurbach, et al., “Rechargeable Magnesium Batteries,” US Pub. No. 2008/0182176, Jul. 31, 2008. (This reference discloses a Chevrel phase intercalation compound as a cathode active material.)    (5) R. E. Doe, et al., “Rechargeable Magnesium-ion Cell Component and Assembly,” US Pub. No. 2011/0159381, Jun. 30, 2011. (This reference reported that glassy carbon, graphite foil, and graphite fibers have a high anodic stability limit and, hence, suggested the use of these materials as a current collector, not as an anode active material or cathode active material. The anode active material used was Mg metal (hence, this reference really teaches about a Mg cell, not Mg-ion cell) and there was no teaching about an anode active material. The cathode active materials suggested in this reference were those prior art intercalation compounds, such as Chevrel phase and spinel compounds.)
The current cathode active materials proposed for use in rechargeable Mg batteries and the current Mg batteries as a device or as an industry sector have the following serious drawbacks:                (1) The practical capacity achievable with current cathode materials has been mostly lower than 300 mAh/g and more often less than 150 mAh/g.        (2) The production of these cathode active materials normally has to go through a high-temperature sintering procedure for a long duration of time, a tedious, energy-intensive, and difficult-to-control process.        (3) The insertion and extraction of magnesium ions in and out of these commonly used cathodes rely upon extremely slow solid-state diffusion of Li in solid particles having very low diffusion coefficients, leading to a very low power density (another serious problem of today's Mg-ion batteries).        (4) The current cathode materials are electrically and thermally insulating, not capable of effectively and efficiently transporting electrons and heat. The low electrical conductivity means high internal resistance and the necessity to add a large amount of conductive additives, effectively reducing the proportion of electrochemically active material in the cathode that already has a low capacity. The low thermal conductivity also implies a higher tendency to undergo thermal runaway, a major safety issue in lithium or magnesium battery industry.        (5) Most of these cathodes contain a high oxygen content that could assist in accelerating the thermal runaway and provide oxygen for electrolyte oxidation, increasing the danger of explosion or fire hazard. This is a serious problem that has hampered the widespread implementation of lithium-ion batteries in electric vehicles (EVs). If not properly addressed, this problem will also impede the use of Mg-ion batteries in EVs.        (6) A potentially good cathode active material disclosed in open literature is graphite fluoride (GF) [e.g., J. Giraudet, et al., “Magnesium Batteries: Toward a First Use of Graphite Fluorides,” Journal of Power Sources, 173 (2007) 592-598]. The GF, used as a cathode active material, exhibits a specific magnesium storage capacity C of up to 572 mAh/g based on the cathode active material weight. With a medium operating voltage V112 of 1.08 volts, the maximum specific energy (also based on the cathode active material weight) is Es=C*V1/2=618 Wh/kg (Table 1 of Giraudet, et al). This is approximately equivalent to a cell-level specific energy of 618/5=124 Wh/kgcell, which is lower than those (120-180 Wh/kgcell) of most of the current Li-ion cells. Further significantly, the best specific power of these Mg-ion cells featuring a GF cathode is only 10.8-12.0 W/kgcathode or 2.2-2.4 W/kgcell, which is 2-3 orders of magnitude lower than the specific power of current Li-ion cells. The batteries for various EVs demand a specific power significantly higher than 1,000 W/kgcell. Clearly, as of now, even the best Mg-ion cells fall short of the performance requirements of EV batteries by a huge margin.        (7) There are only an extremely limited number of electrolytes known to be potentially suitable for use in a rechargeable Mg cell; none of them are commercially available. This has been the most severe impediment to the practical use or commercialization of rechargeable Mg batteries. Most of the presumably good candidate electrolytes induce a thick and dense passivating layer on the Mg metal at the anode. This passivating layer is impermeable to Mg ions and electronically insulating, effectively preventing Mg dissolution (during cell discharge) and Mg ion re-deposition (during cell recharge).        
Hence, it is an object of the present invention to provide a high-capacity cathode active material (preferably with a specific capacity greater than 250 mAh/g) for use in a magnesium-ion cell.
It is another object of the present invention to provide a high-capacity Mg-ion cell featuring a cathode active material that exhibits a specific capacity greater than 300 mAh/g, preferably greater than 600 mAh/g, or more preferably greater than 800 mAh/g, leading to a cell-level specific energy greater than 250 Wh/kgcell, typically greater than 300 Wh/kgcell, or even greater than 600 Wh/kgcell.
It is still another object of the present invention to provide a Mg-ion cell having a high-capacity cathode active material (with a specific capacity significantly greater than 250 mAh/g) that can be readily prepared without going through an energy-intensive sintering process.
Another object of the present invention is to provide a high-capacity cathode active material (with a specific capacity greater than 250 mAh/g) that is capable of storing magnesium atoms without the need to undergo magnesium intercalation, thereby leading to a significantly improved power density and reduced recharge time.
Yet another object of the present invention is to provide a high-capacity cathode active material that is electrically and thermally conductive, enabling high-rate capability and effective heat dissipation in a Mg-ion cell.
It is still another object of the present invention to provide a high-capacity cathode active material that contains little or no oxygen, reducing or eliminating the potential fire hazard or explosion.
It is a further objective of the present invention to provide Mg-ion cells that can operate on a much wide scope of electrolytes.
It is an ultimate object of the present invention to provide a high energy density and high power density magnesium-ion cell featuring a high-capacity cathode active material that does not operate on magnesium intercalation, is intrinsically conductive (both thermally and electrically), contains little or no oxygen, can be fabricated cost-effectively and without consuming lots of energy, and exhibit a specific capacity significantly greater than 300 mAh/g.